1. Field of the Invention
The present invention relates to a lens holding apparatus and a light-beam scanning optical apparatus, and more particularly to a lens holding apparatus and a light-beam scanning optical apparatus for use as an image writing means in a laser printer or digital photocopier.
2. Description of the Prior Art
Various types of light-beam scanning optical apparatus are known that are used as means for writing images onto a photoreceptor in electronic photography apparatus. Typically, in such scanning optical apparatus, a light beam emitted from a light source unit is deflected by a polygon mirror, and is then focused, through optical members such as a scanning lens, on a photoreceptor to scan the surface thereof. In conventional light-beam scanning optical apparatus, to compensate for the tilt error of each deflecting surface of the polygon mirror, a condenser lens (usually a cylindrical lens) is provided between the light source unit and the deflector (polygon mirror) so that the light beam will be focused, only in the sub-scanning direction, in the vicinity of the deflecting surface, and another lens member is provided between the deflector and the photoreceptor so that the light beam will be focused, in the sub-scanning direction, on the photoreceptor. In addition, to alleviate degradation of produced images (resulting from uneven pitches in the sub-scanning direction) due to the tilt error of the deflecting surface, it is customary to set the absolute value of the magnification of the scanning lens in the sub-scanning direction to 2x or less.
However, setting the magnification in the sub-scanning direction to so small a value reduces the beam-waist diameter obtained on the photoreceptor that serves as the image plane of the scanning optical system, and, inconveniently, this makes the depth of focus unduly short. As a result, even a slight amount of defocus arising in the scanning optical system due to a change in ambient conditions causes the beam-waist diameter to vary considerably, and thus, in conventional scanning optical apparatus, the quality of produced images tends to vary with ambient conditions.
Accordingly, in conventional light-beam scanning optical apparatus, the shape of the light beam on the photoreceptor is corrected into a desired shape either by using a flat aperture, sacrificing efficient use of light, or by using a long-focal-length cylindrical lens, sacrificing compactness of the apparatus.
In order to overcome the above-noted problems, in Japanese Laid-open Patent No. H9-97065, a light-beam scanning optical apparatus is proposed which provides a sufficiently small beam-waist diameter on the photoreceptor despite having a relatively short optical path and that automatically corrects the beam-waist diameter in accordance with the ambient temperature. In this light-beam scanning optical apparatus, the use of a cylindrical lens composed of two lenses, i.e. a first lens that is a convex lens and a second lens that is a concave lens, helps to bring the principal point of the cylindrical lens that corresponds to the image formed on the deflecting surface of the polygon mirror closer to the light source, and thus to make the composite focal length of the first and second lenses shorter.
However, the light-beam scanning optical apparatus proposed by the above-mentioned Japanese Laid-open Patent No. H9-97065 requires the beam-waist diameter on the photoreceptor to be adjusted through the adjustment of each of the lenses constituting the cylindrical lens in terms of the direction of the optical axis and the rotational angle about the optical axis, and thus requires a complicated adjustment mechanism as well as a complicated adjustment method.